Coherent demodulation and differentially coherent demodulation, or simply differential demodulation, are conventional techniques used for retrieving data conveyed by an incoming carrier signal. Between the two techniques, coherent demodulation typically achieves a 1-3 dB improvement in performance. Generally, during each symbol coherent demodulation retrieves data from a carrier signal by comparing a phase relationship between the signal's quadrature components with an absolute phase reference. Noise influences the carrier signal's phase but not the absolute phase reference. Differential demodulation uses the carrier phase from a previous symbol as a reference. Thus, noise influences both the carrier signal's phase and the reference in differential demodulation. This doubling of the noise influence causes differential demodulation to exhibit poorer performance than coherent demodulation. Consequently, digital communication systems benefit from using coherent demodulation.
However, differential demodulators have conventionally been much simpler to implement than coherent demodulators and have conventionally been able to generate valid data quicker than coherent demodulators. One reason for the simplicity of and rapid acquisition by differential demodulators is that coherent demodulators need to generate an absolute phase reference while differential demodulators have no such requirement. In conventional coherent demodulators, the generation of an absolute phase reference requires time and the use of complex components. Extensive carrier phase acquisition time is required because phase locked loops with typically narrow bandwidths must first achieve a locked condition before valid data may be recovered.
This extensive carrier-phase acquisition time is exacerbated by a "hang-up" phenomenon. Hang-up occurs when an initial phase locked loop Oscillator's phase is nearly 1/2 way between two of the discrete phase states the carrier signal uses to convey data. In hang-up, the phase locked loop has trouble deciding which direction to adjust its oscillator, and carrier phase acquisition may experience unusual and serious delays as a result.
In addition, the phase locked loop components which generate the absolute phase reference have conventionally been the most complex circuits in the entire demodulator. In older implementations of coherent demodulators, the carrier phase acquisition loop included analog components which suffer from well known offset, drift, and noise problems that are not present when digital implementations are constructed. In improved implementations of coherent demodulators, the carrier phase acquisition loop may be implemented using only digital components. Since they do not suffer from the well-known analog implementation problems, they are generally regarded as being more reliable.
However, conventional digital carrier phase acquisition loops usually include numerically controlled oscillators (NCOs) and full complex multipliers to perform full quadrature multiplication. The NCO and full complex multiplier together typically account for well over half of the gates utilized in the entire demodulator. Since power consumption is proportional to the number of gates involved, a considerable amount of power is consumed by the carrier phase acquisition loop. Power consumption is also typically proportional to the speed of operation. Thus, for a given power consumption or dissipation level the excessive complexity of a conventional carrier phase acquisition loop leads to reduced symbol rates. Alternatively, for a fixed symbol rate the excessive complexity of a conventional carrier phase acquisition loop leads to increased power consumption.
Further, conventional demodulators, whether differential or coherent, compute symbol synchronization timing errors by examining quadrature component signals only near their zero-crossing points. Typically, symbol synchronization occurs when sampling points are driven as far from the zero-crossing points as possible. Unfortunately, this technique leads to sub-optimum operation in channels for which a maximum eye opening is not centered between zero-crossing points.
In addition, conventional demodulators yield an undesirably slow symbol acquisition time. The slow acquisition time, for both differential and coherent demodulators, is particularly burdensome in burst communication systems. Such systems include overhead preamble data in each burst to aid symbol and/or carrier phase synchronization. Unfortunately, the preamble reduces the space available for the burst's payload data, and conventional demodulators often require undesirably long preambles for the purpose of acquiring symbol timing.